The present invention generally relates to a communication system, and more particularly to a communication system for efficiently starting a teleconference between two multimedia stations over a network.
CCITT has recommended various standards of multimedia communication equipment and various standards of data transmission protocols for transmitting data over an ISDN (Integrated Services Digital Network).
FIG.1 shows a model of data transmission protocols used by a multimedia communication system to carry out a teleconference between two multimedia stations over an ISDN. The ISDN which is a communication path between the two multimedia stations is composed of a D channel (a signal channel) and a plurality of B channels (information channels). The layer-1 in the reference model of OSI (Open Systems Interconnection) for each channel is constructed in conformity with the CCITT 1.430 standard.
The D channel is used to perform a transport connection between the two multimedia stations after the processing of the layer-2 and the layer-3 in the reference mode of OSI is performed. The audio signal, the video signal, the MLP (Multi-Layer Protocol) signal and the BAS (Bit rate Allocation Signal) are multiplexed and the multiplexed signal is transmitted over ISDN by using the B channels in conformity with the CCITT H.221 standard.
The audio signal indicates the voice during a teleconference between two multimedia stations, and the video signal indicates an image produced by a video camera during a video conference between the stations. The MLP includes a CI (control and indication) signal and a TW (telewriting) signal,, The CI signal is used to start and stop a transmission control procedure for TW transmission. The TW signal indicates the control of a manually-input drawing and the control of displayed screen data. In accordance with the CCITT X.225, X.224 and Q.922 standards, the CI signal and the TW signal are included in the MLP signal, and the MLP signal, including the CI signal and the TW signal, is transmitted over ISDN. The BAS indicates an equipment capability list and control commands, and the BAS signal is transmitted through a procedure in conformity with the CCITT H.242 standard.
FIG.2 shows transmission control procedures performed when a teleconference between two multimedia stations over an ISDN is started. As shown in FIG.2, a start request for transmission is issued by an operator on a source station by setting the identification code of a destination station and sending a transmission start command to the destination station.
After the destination station is called from the source station, a first channel connection protocol between the two stations is run to acquire a B channel of the ISDN. The B channel, after it is acquired, enables the transmission of an audio signal from one station to the other and vice versa, so that the operators at the two stations can talk with each other.
After the two stations have received an equipment capability list from the other station, a second channel connection protocol between the two stations is run to acquire another B channel. After the two B channels are acquired, the AUDIO+VIDEO mode is set so that the audio signal and the video signal can be transmitted through the two B channels. The operators at the two stations can talk with each other and can view an image produced by a video camera of the other station.
In order to start a telewriting (TW) communication between the two stations after the AUDIO+VIDEO mode is set, it is necessary for at least one of the source station and the destination station to issue a TW connection start command to the other station. In FIG.2, the operator on the source station issues a TW connection start command to the destination station, so that the mode of each of the two stations is changed to an AUDIO+VIDEO+MLP mode.
After the AUDIO+VIDEO+MLP mode is set for the two stations, the source station sends a CI connection request to the destination station over the network. A CI connection protocol between the two stations is run, so that a CI connection between the two stations is thus established.
After the CI connection is established, the source station sends a TW connection start request to the destination station over the network. After a response from the destination station is received, the source station sends a TW connection request to the destination station over the network. A TW connection protocol between the two stations is run, so that a TW connection between the two stations is thus established. The MLP transmitting channel acquired by the TW connection enables the starting of a TW communication between the two stations over the ISDN.
After the TW connection is established, if one of the operators on the two stations manually inputs a drawing, the manually-input drawing is transmitted from one station to the other station over the network. For example, the operator on the destination station can view the drawing supplied from the source station on a display monitor on the destination station. Also, if the operator on the source station performs a page break on a display monitor on the source station, the display control data is transmitting to the destination station over the network. The display monitor on the destination station is subjected to a page break in synchronism with the page break of the display monitor on the source station. In this manner, a TW communication between the two stations over the ISDN is carried out.
In order to end the TW communication between the two stations, it is necessary for at least one of the two stations to issue a TW disconnection request command to the other station. In FIG.2, the operator on the source station issues a TW disconnection request to the destination station. After a response from the destination station is received, the source station sends a TW release request to the destination station. A TW connection releasing protocol between the two stations is run, so that the TW connection between the two stations is released. The source station further sends a CI release request to the destination station. A CI connection releasing protocol between the two stations is run, so that the CI connection between the two stations is released. Then, the teleconference between the two stations over the ISDN ends. A given disconnection procedure between the two stations is later performed, and the first and second channel connections between them are also released.
Two methods for setting the abilities of the stations to issue the teleconference start request are conceivable: only one of the two stations is allowed to issue a teleconference start request; or both the two stations are allowed to issue teleconference start requests. If only one of the two stations is allowed to issue a teleconference start request, the non-allowed station cannot issue a teleconference start request at all times.
On the other hand, if both the two stations are allowed to issue the teleconference start requests, there is a problem in that the two stations may issue the teleconference start request to each other at the same time. In such a case, an MLP connection between the two stations fails due to the simultaneously issued requests of the two stations. FIG.3 shows a portion of the transmission control procedures when the two stations have issued the teleconference start request to each other at the same time.
In FIG.3, after the AUDIO+VIDEO+MLP mode is set, the source station issues a CI connection request to the destination station, and, at the same time, the destination station issues a CI connection request to the source station. The CI connection request is transmitted from one station to the other station over the network in conformity with of the procedure of the CCITT X.225 standard. Since the case in which the two stations have issued the teleconference start requests at the same time is not taken into consideration by the CCITT X.225 standard, a connection error occurs in the above case. The transmission control procedure is thus stopped, and the CI connection fails.
In order to start running the CI connection protocol between the two stations after a connection error is detected, it is necessary for the operator on one of the two stations to re-issue a teleconference start request and then send a CI connection request to the other station. If the two stations have again issued the teleconference start request to each other at the same time, the operator on the above station has to repeat the same operations as he previously performed.
Therefore, if both the two stations are allowed to issue the teleconference start request, the two stations may issue the teleconference start request to each other at the same time. The two connection requests, simultaneously issued from the two stations, conflict with each other, causing a connection error to occur. It is necessary for the operator on one of the two stations to perform a teleconference starting operation again whenever a connection error is detected. This is considerably inconvenient to the operators on the two stations.